The present invention relates generally to lasers, and more particularly to dye lasers suitable for medical therapies such as selective photothermolysis and photodynamic therapy.
Many medical procedures employing lasers require relatively long pulse durations at specific wavelengths to achieve optimal results. Therapies such as laser removal of hair and vascular and pigmented lesions rely upon the selective photothermolysis of blood vesicles and/or cells. The principles of selective photothermolysis were first described by Anderson and Parrish in xe2x80x9cSelective Photothermolysis: Precise Microsurgery by Selective Absorption of Pulse Radiationxe2x80x9d, Science, Vol. 220, pp. 524-27 (1983).
To provide selective photothermolysis in an efficient manner, the following three criteria must be satisfied:
(1) The target tissue must absorb more of the incident laser light than the adjacent tissue;
(2) The intensity of the laser light and the absorption of the target tissue must be sufficiently high to coagulate, kill, or vaporize the target tissue, and;
(3) The pulse duration of the laser light must be short enough to avoid or minimize overheating of adjacent tissue due to thermal diffusion from the target tissue, but long enough to prevent explosive boiling of the target and/or adjacent tissue.
The foregoing criteria are generally satisfied by matching the laser pulse duration and wavelength to the thermal and spectral characteristics of the target tissue. In order to accommodate targets of varying size and absorption coefficients, it is necessary to adjust pulse duration and wavelength over a fairly wide range.
To date, prior art lasers used or intended for use in selective photothermolysis and similar procedures have fallen into one of two categories: fixed pulse duration, variable wavelength lasers, and fixed wavelength, variable pulse duration lasers. The former category includes long pulse flashlamp pumped dye lasers (known as FLPDLs), various examples of which are disclosed in U.S. Pat. Nos. 5,066,293; 5,287,380; 5,624,435, and; 5,668,824. FLPDLs designed for medical use have maximum pulse durations of approximately 1.5 milliseconds, which limits their use to very small blood vessels. Another problem associated with FLPDLs is that while they allow adjustment of the output beam wavelength, they do not offer the ability to adjust the pulse duration over a clinically relevant range.
Examples of lasers in the second category (fixed wavelength, variable pulse duration lasers) include xe2x80x9cStar Pulsedxe2x80x9d KTP lasers available from Laserscope, Inc. of San Jose, Calif. These lasers employ a pulsed arc lamp to generate high intensity light having a wavelength of 532 nanometers and pulse durations ranging from 1-100 ms. Variable duration long pulse 532 nanometer light can also be generated using flashlamp-pumped lasers, such as the Versipulse laser available from Coherent Laser of Santa Clara, Calif. Although the pulse durations of these lasers render them suitable for treating medium- and large-sized blood vessels, the wavelength of the output beam of lasers of the foregoing description cannot be adjusted to match the spectral characteristics of the target tissue.
In view of the limitations of prior art lasers, there is a need for a medical laser system that offers the ability to adjust both pulse duration and wavelength in order to match thermal and spectral characteristics of the target tissue and thereby achieve highly efficient and effective results.
The present invention provides a medical laser system having an output beam that can be adjusted over a clinically relevant range of pulse durations and wavelengths.
According to one embodiment, the medical laser system includes a solid state laser for generating an input beam of adjustable pulse duration. The solid state laser includes a laser medium, such as a neodymium: yttrium aluminum garnet (Nd:YAG) rod, which is pumped by an excitation source typically comprising an arc lamp. Light emitted by the laser medium is passed through an acousto-optic Q-switch and a frequency doubling non-linear crystal. Power supplied to the excitation source is modulated to produce pulses of a specified duration, each pulse comprising a train of repetitively Q-switched micropulses.
The input beam is coupled to a dye laser, either directly or through an optical fiber. The dye laser generates an output beam of adjustable wavelength having a pulse duration corresponding to the pulse duration of the input beam. The dye laser includes a dye cell onto which the input beam is focussed, and a tuning element, such as a birefringent filter. Adjustment of output beam wavelength is accomplished by changing the dye composition and/or varying the filter orientation. A portion of the output beam may be split off and diverted to power and wavelength detectors, which provide feedback signals to a control processor driving the excitation source, Q-switch and tuning element. A conventional delivery system, which may comprise an optical fiber and associated focussing optics, is coupleable to the dye laser and serves to direct the output beam onto a biological tissue target.
In accordance with certain embodiments of the medical laser system, the wavelength of the output beam produced by the dye laser may be varied between 550 nanometers and 750 nanometers, and the pulse duration varied between 0.1 and 900 milliseconds. A clinician operating the laser may thus adjust the beam characteristics in order to achieve optimal selective photothermolysis of the target tissue. The medical laser may also be advantageously used for a variety of other therapies and procedures, including hair removal, drug activation in photodynamic therapy (PDT), and cutting and/or drilling tissue.